This invention relates to an electrical bushing for use with a gas insulated electrical apparatus, and more particularly to an electrical bushing in which the concentration of an electric field is moderated by a grounded shield and an intermediate shield.
FIG. 1 illustrates one example of a conventional electrical bushing for use with a gas insulated electrical apparatus which is disclosed in Japanese Utility Model Application Laid Open No. 56-71007.
The conventional electrical bushing comprises an insulating tube 1 such as a porcelain tube, an upper mounting fixture 12 secured to one end of the tube 1, an upper terminal 11 hermetically secured to the tube 1 through the fixture 12, a lower mounting fixture 10 secured to the other end of the tube 1, a flange 5 which is grounded and hermetically secured to the insulating tube 1 through the mounting fixture 10, and a grounded vessel 7, having a flange member 7a at the top end thereof and which is hermetically secured to the flange 5.
The electrical bushing further comprises a central conductor 2 axially disposed at the center of the hollow portion of the tube 1 to be used as a high voltage electrode, and grounded hollow cylindrical shield 3, having a field moderating ring 3a formed at one end thereof and fixed at the other end thereof to the inner circumferential edge of the flange 5 along the inner wall of the tube 1. An intermediate cylindrical shield 4 is disposed between the central conductor 2 and the cylindrical grounded shield 3 so as to surround the central conductor 2, and is supported in the tube 1 by an insulator 6 which is fixed at one end thereof to the flange 5 and at the other end thereof to the intermediate shield 4 for supporting the shield 4. An insulating gas 8, such as SF.sub.6 gas, is filled within the electrical apparatus.
In the conventional electrical bushing as above described, the voltage distribution in the inner space of the tube 1 is determined by the ratios of the electrostatic capacity between the central conductor 2 and the intermediate shield 4, and that between the grounded shield 3 and the intermediate shield 4. In determining the electric field in the insulating gas 8, it is necessary to consider the balance of the electric field between the electric field moderating ring 3a of the grounded shield 3 and a portion of the central conductor 2 which is surrounded by the intermediate shield 4.
Further, it has been recently attempted to minimize the dimensional size of the insulating tube 1 so that the production cost of the tube may be cut down. Accordingly, the inner space of the tube is very limited, so that the insulating distance between the field moderating ring 3a of the grounded shield 3 and the intermediate shield 4 becomes shorter.
The conventional electrical bushing described above has problems in that, when a high voltage is applied to the central conductor 2, the electric field is considerably higher around the field moderating ring 3a of the grounded shield 3, resulting in a limitation to the withstand voltage characteristics thereof. In this context, the field moderating ring 3a must have a cross-sectional shape having as large a radius as possible to moderate the concentration of the electric field, and, as a result, the inner diameter of the tube 1 has to be enlarged, contradicting the minimum size and production cost requirements.